The following background information is provided to assist the reader to understand the environment in which the invention will typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless specifically stated otherwise in this document.
Friction type draft gear assemblies widely used in United States railway industry to provide protection to a railway car by absorbing shocks in both draft and buff conditions must meet various Association of American Railroads (AAR) requirements. In one aspect draft gear must be capable of maintaining the minimum shock absorbing capacity during its service life required by AAR standard M-901-G to be at least 36,000 foot pounds being measured during a drop hammer test. In the other aspect AAR mandates working action of such draft gear to be achieved without exceeding a 500,000 pound reaction pressure acting on the freight car sills in order to prevent upsetting the coupler shank. In a further aspect, the draft gear must fit into a standard AAR railway car pocket of 24.625 inches in length.
The commonly used draft gears, installed in alignment with a railway car center sill, include a housing having an open front and a closed rear portions. A compressible cushioning means is positioned within the rear portion of the housing. A friction cushioning element is adopted in the front portion of the housing. The draft gears further include a spring release mechanism for continuously urging the friction cushioning element outwardly from the compressible cushioning means thereby releasing such friction cushioning element after compression of such draft gears. The compressible cushioning means is typically either of an all spring configuration as taught by U.S. Pat. Nos. 5,152,409 and 5,590,797, of a spring and hydraulic assembly combination as taught by U.S. Pat. No. 3,368,698, or of an elastomeric pad stack as taught by U.S. Pat. Nos. 6,488,162 and 6,446,820.
All standard steel housing draft gears presently fitting in the AAR standard 24.625-inch pocket have a nominal 3.25-inches of travel. This travel applies to each buff and draft conditions for a total travel distance of 6.50-inches. This 6.50-inches of total travel is spelled out in the various M-901 specifications. The only exception presently on the market is an all elastomeric device called the Twin-Pac as taught by U.S. Pat. No. 6,446,820. This design uses a housing with integral yoke and two different stacks of elastomeric pads divided by a spacer plate. In draft only the front stack of pads is loaded, however in buff both stacks of pads are loaded. This design allows for a different amount of draft and buff travel but still falls within the 6.5-inches of total travel required by the specification. Because the resilient compression members in the Twin-Pac draft gear are made exclusively from elastomeric material, the design has several drawbacks, including high recoil forces that cause the cars to bounce back and forth during an impact. This bouncing is also possible in normal train line action. Additionally, testing indicated that such draft gear assembly was damaged during testing when subjected to a higher buff impact loads which are applicable during operation of the railway vehicle.
Therefore, in the first aspect, it is desirable to provide a friction draft gear assembly with an integral yoke portion having a more reliable operation.
In most, if not all cases, impact energy levels during buff conditions are much higher than the train line energy levels during draft conditions. Therefore, in the second aspect, it would be appreciated that any device used to control energy should be able to absorb more energy in buff rather then in draft while fitting into AAR standard 24.625-inch pocket.